1. Field of the Invention
This invention relates to a SiC single crystal and a method for the growth thereof. More particularly, this invention relates to a method for the growth of a SiC single crystal of good quality which is useful in the field of applications such as to a short wavelength operated light-emitting diode or to an electrical device, and useful as a seed crystal for growth of another SiC single crystal ingot.
2. Prior Art
The SiC single crystal is a material physically and chemically stable and capable of withstanding the impacts of high temperature and radiation and, therefore, is expected to find utility as a material for semiconductor elements resistant to the impacts of environment. Further, the SiC single crystal has already found utility as a material for light-emitting diodes operating with short wavelengths owing to a wide bandgap. In fact, the 6H--SiC has bandgap of about 2.9 eV at normal room temperature and, therefore, serves favorably as a material for blue light-emitting diodes. The 4H--SiC has bandgap of about 3.1 eV and, therefore, serves favorably as a material for purple light-emitting diodes.
The SiC single crystal ingot is produced by the sublimation-recrystallization method. As the seed crystal for this production, a SiC single crystal substrate having a {0001} face (c face or the basal plane) thereof exposed has been mainly used heretofore. In this case, since the seed crystal does not always induce growth of a polytype identical thereto, it has been customary to attempt the growth of a polytype aimed at by optimizing the conditions of growth such as temperature and pressure. Japanese Unexamined Patent Publication No. 48,495/1990 discloses a method for the growth of 4H--SiC single crystal. The specification of this publication has a mention to the effect that 6H polytype crystal grows on the lower temperature side and 4H polytype crystal grows on the higher temperature side respectively of the neighborhood of 2,250.degree. C. as the seed crystal temperature. By this method, the control of growth for the formation of the polytype aimed at cannot be effected completely and the produced crystal tends to suffer from degradation of crystallinity owing to the mixture of other polytype.
Data EFM-88-24, p. 24, published by Electric Science Society (Japan), Electronic Material Study Group, on Sep. 5, 1988 has a report on the evaluation of a SiC single crystal obtained by the method mentioned above. The report reads that when the SiC single crystal is etched with fused KOH to unveil defects contained in the crystal, numerous etch pits presumably corresponding to dislocations appear, that these etch pits are hexagonal pits grouped into three sizes, i.e. 10.sup.2 to 10.sup.3 of large size/cm.sup.2, 10.sup.4 of medium size/cm.sup.2, and 10.sup.5 of small size/cm.sup.2 as arranged in the decreasing order of size, and that these etch pits correspond to dislocations or linear defects. Particularly, the defects which correspond to large etch pits form pinholes piercing the crystal and constitute themselves a cause such as for leak current in a device to be made of the crystal. The defect is called as an "empty tube" or "micropipe" defect.